This invention generally relates to hydrophilic polymeric foam materials which can be placed in compression for an extended period of time and then rapidly re-expanded in contact with water or moisture. This invention is particularly concerned with re-expandable hydrophilic foams possessing utility in catamenial devices such as a tampon assembly.
Polymeric foam materials have been compressed and subsequently re-expanded by various methods in the art. One such method comprises treating the foam with a suitable adhesive to maintain the foam in a compressed state, and, when desired, removing the adhesive by means of a solvent to re-expand the foam. Another common method comprises incorporating a material, solid at normal temperatures but liquid at elevated temperatures, into the foam. In this technique, the material is impregnated into the foam as a liquid, the foam compressed and subsequently cooled. Upon reheating the foam to the range of the melting point of the impregnated material, the foam then re-expands.
The above techniques both possess the shortcoming that the re-expansion of the foam is comparatively slow. That is, in the uses to which such foams have largely been directed, no requirement has existed that the foam re-expand rapidly. In addition, foam materials held in compression by the above methods, and thereby reduced in volume for extended periods of time, tend to remain at the reduced volume even after the compressive force is removed. This condition is known as compression set or fatigue, and is particularly deleterious in the instance where the foam is to serve in a liquid absorbing capacity, such as in a catamenial tampon assembly, and where the useful volume of the foam must, if anything, exceed that of the original shape. If employed in a tampon, the foam must re-expand as rapidly as possible to prevent the escape of menstrual fluids from the vagina, a phenomenon known as bypass. Reliance either upon body fluid as a solvent for an adhesive, or the use of body heat to melt an impregnated agent is too time consuming. Too much time is required for body moisture and/or body heat to fully permeate the compressed foam in order to cause re-expansion.
An additional method to provide a compressed, re-expandable foam is disclosed in U.S. Pat. No. 3,306,966, to Matejcek et al. In this reference, the foam is impregnated with a defatiguing agent and a fixating agent. The defatiguing agents are liquid, water soluble substances such as polyethylene glycol. The foam is either coimpregnated or coated with a fixating agent. The fixating agent maintains the compressed foam containing the defatiguing agent in the compressed condition until it is dissolved away or its effect removed by exceeding its melting point. The function of the defatiguing agent is to aid the foam in regaining its original volume when later re-expanded. While the foams so treated tended to exhibit reduced fatigue or compression, the regularity of extent of expansion particularly required for catamenial purposes was lacking. This was believed to be due to the water solubility of the fatiguing agent, which failed to be uniformly entrained by the foam during impregnation, and could be easily expelled during the removal of excess solution.